Wouldn't this also be correct? "János egy okos és ügyes fiú, de Péter hülye!"
That is okay. You could also say "de Péter egy hülye" but that wouldn't match the English sentence anymore.
It is now accepted 10 October 2017.
Would "Péter pedig hülye" work here?
Not really. That would relate to an "and" in the English sentence, instead of the "but".
What about képes for "able": wouldn't that also be correct?
It sounds good. At the least it would be a better translation for "able". Ügyes is more along the lines of "dexterous" or "skilful". (What, in British English "skillful" has just a single l? Jeesh.)
Cheers for both the answer and the new info on BE ;)